 Physical inactivity has been called the biggest public health problem of the 21st century. Of course, just because someone calls it that doesn't mean it's true. In fact, physical inactivity ranks down at number 5 in terms of risk factors for death, and number 6 in terms of risk factors for disability. Diet is by far our greatest killer, followed by smoking. But still, there is irrefutable evidence of the effectiveness of regular physical activity in the prevention of several chronic diseases, cardiovascular disease, diabetes, cancer, hypertension, obesity, depression, and osteoporosis, as well as premature death, adding an additional one or two years onto our lifespan, helping to add years to our life, and above all, life to our years. It truly may be survival of the fittest. How much do we need to exercise? In general, the answer is the more the better. Currently, most health and fitness organizations advocate a minimum of 1,000 calories burned of exercise a week, which is like walking an hour a day, five days a week. But seven days a week may be even better in terms of extending one's lifespan. Hard intensity can be practically defined by the talk-but-not-sing test, where you can still carry on a conversation, but would feel breathless if trying to sing. Exercise is so important that not walking an hour a day is considered a high-risk behavior alongside smoking, XX drinking, and being obese. Having any one of these effectively ages us three to five years in terms of risk of dying prematurely. Though interestingly, those that ate green vegetables on a daily basis did not appear to have that same bump in risk. But even if broccoli-eating couch potatoes do live as long as walkers, there are a multitude of ancillary health benefits to physical activity. So much so that doctors are encouraged to prescribe it, to signal to the patient that exercise is medicine. In fact, powerful medicine. Researches at London School, Harvard, and Stanford compared exercise to drug interventions and found that exercise often worked just as well as drugs for the treatment of heart disease and stroke, the prevention of diabetes. Of course, there's not a lot of money to fund exercise studies, so one option would be to require drug companies to compare any new drug to exercise. In cases where drug options provide only modest benefit, patients deserve to understand the relative impact that physical activity might have on their condition. We could throw diet into the mix, too. Yes, the FDA could tell drug companies, your new drug beats out placebo, but does it work as well as kale?